1. Field of the Invention
This invention relates to a resin composition suitable for a toner. More particularly, it relates to a resin composition suitable as a binder for an electrophotographic toner.
2. Description of the Prior Art
In electrophotography (xerography), for fixing electrostatic latent images visualized by use of a developing toner, there have been widely used methods using fixing means of contact heating, such as those with a heated roller and those via a film between a heater and paper or the like (for example, JPN Patent Lay-open Nos. 70688/1992 and 12558/1992). In these methods, it is desired that the minimum temperature for fixing (hereinafter referred to as MFT) is low (low temperature fixing properties) and the temperature causing offset to the heated roller (hereinafter referred to as HOT) is high (anti-hot offset properties). Thermal shelf stabilities is also desired so as not to cause coagulation (or agglomeration) and reduction of flowability under heat evolved from fixers within electrophotographic machines.
In order to meet these requirements, there have been heretofore proposed various techniques using toner binders having a wide range of molecular weight distribution from lower molecular weight to higher molecular weight and having a glass transition temperature (hereinafter referred to as Tg) of 50.degree.-80.degree. C. (for example, JPN Patent Publication No. 20411/1985 and JPN Patent Lay-open No. 215558/1986), and those using toners mainly composed of heat reaction products of non-crystallizable polyester resins with vinyl copolymer resins (for example JPN Patent Lay-open No. 277074/1990), those using toners composed of a vinyl resin as a discontinuous domain and a polyester resin as a continuous matrix, and a dispersing diameter of vinyl resin of at most 5 .mu.m (U.S. Pat. No. 5,250,382), those using toners composed of a vinyl resin as a continuous matrix and a polyester resin as a discontinuous domain, and a dispersing polyester resin with a diameter of at least 0.5 .mu.m (JPN Patent Lay-open No. 260062/1985), and those using toners prepared by solution copolymerization of styrenic monomers with acrylate monomers in the presence of saturated polyester resins (JPN Patent Publication No. 25057/1989).
In these techniques, there are drawbacks, that the formers cannot sufficiently answer to fixing properties at lower temperature required in recent high speed facsimile or copy machines, or to higher thermal shelf stability desired accompanied with miniaturization of printers; and that the latter results in poor dispersability of colorants and charge controllers, insufficient frictional charge amount, libility of vinyl resin bleeding out of the polyester resin matrix to toner surface, and difficulty in viscosity reduction.
Toners disclosed in U.S. Pat. No. 5,250,382, result in a difficulty in viscosity reduction.
Toners controlled by a dispersing diameter of polyester resin of at least 0.5 .mu.m by mixing time of toners, disclosed in JPN Patent Lay-open No. 260062/1985, results in poor low temperature fixing properties.
Toners with uniform dispersing of polyester by copolymerization of styrenic monomers with acrylate monomers in the presence of a solvent and a saturated polyester which is not dissolved in styrenic monomers or acrylate monomers, disclosed in JPN Patent Publication No. 2507/1989, result in poor low temperature fixing properties.
Accordingly, a toner resin with both a low MFT and a high HOT is still being sought.